bug#65051: internal_equal manipulates symbols with position without checking symbols-with-pos-enabled.

[Eli Zaretskii]

> Date: Sat, 5 Aug 2023 11:52:52 +0000
> Cc: 65051@debbugs.gnu.org, Stefan Monnier <monnier@iro.umontreal.ca>,
>   acm@muc.de
> From: Alan Mackenzie <acm@muc.de>
> 
> Hello, Eli.
> 
> On Sat, Aug 05, 2023 at 13:57:50 +0300, Eli Zaretskii wrote:
> > > Date: Sat, 5 Aug 2023 10:45:37 +0000
> > > Cc: 65051@debbugs.gnu.org, acm@muc.de
> > > From: Alan Mackenzie <acm@muc.de>
> 
> [ .... ]
> 
> > > > So we can have two different copies of #<symbol foo at 42>, such that
> > > > their hex values are different?  Isn't that a bug? why don't we
> > > > conflate such identical symbols?
> 
> > > No, it's not a bug, anymore than having two `equal' copies of '(a b c)
> > > would be a bug.
> 
> > But with symbols we store them in obarray, and obarray ought to find
> > the existing slot for #<symbol foo at 42> and reuse it, rather than
> > create a new slot?
> 
> No, only the bare symbol is in the obarray.  The symbol with position
> itself is a pseudovector, with contents (i) a bare symbol (a Lisp_Object
> "pointing at" the obarray) and (ii) the unsigned integer position.

Please document this factoid in the ELisp manual, I think it's very
important, and having it undocumented is a Bad Thing.